Salivary gland acinar cells are extremely sensitive to ionizing radiation (IR). Theories to explain this radiosensitivity mostly focus on increased apoptosis or damage to stem cells and inability to regenerate. However, we have recently demonstrated that maintenance of salivary gland acinar cells is dependent on self-duplication, rather than on stem cell differentiation. This suggests an alternative scenario. We hypothesize that acinar cells are lost due to mitotic catastrophe, i.e. their inability to complete mitosis because of DNA damage. This is consistent with reports that loss of cells to apoptosis is only transiently increased in response to IR and does not account for the ultimate near complete ablation of the acinar cells. Although the underlying mechanism of acinar cell radiosensitivity is not known, radioprotection of salivary glands has been demonstrated by several means. Amifostine is a free radical scavenger that limits the effects of ROS, thereby protecting DNA from damaging strand breaks. Amifostine preserves salivary secretion and some viable acinar cells, but little is reported regarding the molecular status of the protected cells. Recently we developed a radioprotective strategy based on the transient knockdown of Pkc?, a pro-apoptotic gene normally activated by IR. This approach reduced apoptosis, protected acinar cells and preserved saliva secretion in IR-treated glands. However, the long-term consequences of protecting irradiated cells from apoptosis have not been investigated. To be effective, radioprotective strategies must preserve the capacity of acinar cells to self-duplicate, as well as to secrete, while avoiding malignant transformation. In this proposal, we will investigate the hypothesis that radiosensitivity of the salivary glands is due to their inability to self-duplicate (Aim 1). We will address whether siRNA- mediated protection of the salivary glands also rescues the proliferative, as well as secretory function, of acinar cells, without oncogenic effect (Aim 2). We will characterize the protective effects of systemic and localized amifostine to determine the proliferative and secretory capabilities of protected acinar cells (Aim 3). We expect that the information gained from these studies will provide insight into the basic mechanisms involved in acinar cell loss. In addition the planned experiments will validate and further develop experimental approaches for radioprotection. If successful these approaches will lend themselves to translation and the development of novel therapy strategies that would improve the quality of life for survivors of head and neck cancer.